#app .box3 {
  height: 300px;
  width: 300px;
  background-color: burlywood;
}

#app .box4 {
  width: 200px;
  height: 200px;
  background-image: url("./img/big.jpg");
  background-size: cover;
}

#app .box5 {
  width: 100px;
  height: 100px;
  background-image: url("./img/small.png");
  background-size: cover;
}

#test {
  height: 300px;
  width: 300px;
  margin: 100px 0 0 100px;
  /* 加入css3属性 */
  transform: rotate(-45deg);
  background-color: aquamarine;
}